As shown in FIG. 1, an organic light emitting diode (OLED) consists of two electrodes, i.e., anode 20 and cathode 40, which are formed on a substrate 10 having a proper mechanical strength and flatness, and thin-film multilayers of organic material 30 sandwiched therebetween. Such organic light emitting diode is commercially applied in the manufacture of color flat display, and many studies have been recently made on lighting applications of OLEDs.
In general, organic light emitting diode is operated by a phenomenon in which holes and electrons are injected from the anode and the cathode into the organic material, respectively and the recombination of these charge carriers causes light to emit from the device. At this time, a driving voltage is affected by a hole injection barrier height at the interface between the anode material and the organic material, and an electron injection barrier height at the interface between the cathode material and the organic material.
Organic light emitting diode is required to have the properties of high power efficiency and durability. To achieve such properties, the organic material constituting the device has a multilayer structure of a hole injection layer 31, a hole transport layer 32, a light emitting layer 33, and an electron transport layer 34, as shown in FIG. 2, and materials having a new and stable molecular structure have been continuously developed as the organic material constituting each layer.
In the organic light emitting diode having such structure, a light extraction layer is provided on the bottom portion of the substrate to prevent total internal reflection of the light emitted from the device at the interface between the substrate and air. FIG. 3 illustrates an organic light emitting diode that is provided with a light extraction layer 90 on the bottom portion of the substrate.
In particular, the light emitting layer consists of a host material that receives electrons and holes at the same time and a dopant that efficiently converts excitons formed by the recombination of electrons and holes into light. Conventionally, a fluorescent dopant that converts a singlet exciton into light has been used. Recently, a phosphorous dopant that converts a triplet exciton into light is adopted to manufacture a device with high quantum efficiency.
Recently, the present inventors have invented a new operation, in which electrons and holes are generated between the hole injection layer and the hole transport layer, and each of them is transported to the anode and the light emitting layer, instead of injecting holes from the anode into the hole injection layer. This new operation is to generate charges between the organic materials, instead of injecting holes from the anode. Thus, there is no need to overcome the hole injection barrier, and low driving voltage and high stability are also ensured because of using the charges generated at the stable interface.
In order to effectively inject electrons and holes into the organic layers, various materials have been developed as the cathode and anode materials. Organic light emitting diode is fabricated into a device for emitting light through the substrate (bottom-emission) or a device for emitting light in an opposite direction of the substrate (top-emission). The light emitting direction is determined by transmittance of the electrode, through which light passes. In the case of thickly using a material having a high reflectivity such as aluminum, the electrode reflects light. In the case of using a material having high transparency such as metal oxide or a thin metal film having a thickness of passing light, light passes through the electrode. In addition, if both electrodes have high transparency, light can be emitted from both sides.
In organic light emitting diode, the cathode is required to have the properties of readily injecting electrons into the electron transport layer interfaced therewith. The electron injection from the cathode into the electron transport layer is closely related with a difference between LUMO (Lowest Unoccupied Molecular Orbital) level of the electron transport layer and the work function of the cathode materials, and the difference is called an electron injection barrier. The driving voltage of organic light emitting diode depends on the height of the electron injection barrier. The decreased driving voltage is attributed to a low electron injection barrier, and the increased driving voltage is attributed to a high electron injection barrier. Therefore, in order to reduce the height of the electron injection barrier and drive the device at a low voltage, a metal having a low work function is used. The suitable cathode materials include magnesium (Mg), lithium (Li), cesium (Cs), calcium (Ca) or the like, and they can be mixed with other metals for the purpose of improving interfacial adhesion, antioxidant activity, and reflectivity. Since these materials have the work function less than 4 eV, the electron injection barrier, which is the difference between the work function and LUMO level of the electron transport material, is small. On the contrary, metals having the work function more than 4 eV such as aluminum (Al) can be used as the cathode. However, when aluminum is used as the cathode, a higher driving voltage is required because of its high electron injection barrier. To overcome this problem, as shown in FIG. 14, a thin film of an insulating material 41 is interposed between the organic layer and the cathode 42, thereby greatly reducing the driving voltage. Example of the insulating material is represented by lithium fluoride (LiF). Upon applying a voltage to the device, lithium fluoride, which is formed as a thin film having a thickness of 5 to 30 Å, functions to enhance the electron injection from the cathode into the electron transport layer by tunneling, or lithium atoms having a low work function are generated by chemical reaction between lithium fluoride and aluminum deposited thereon, thereby facilitating the electron injection.
The cathode containing a material having a low work function or aluminum is formed by thermal evaporation. In general, a process of forming an electrode using metal or metal oxide is performed by sputtering, e-beam, CVD, thermal evaporation or the like. However, in the manufacture of organic light emitting diode, a process of forming the cathode is performed after the process of forming the anode and organic material. Thus, thermal evaporation, which requires lower energy in the process of forming the cathode, minimizes damage to the organic material that is previously deposited. For this reason, the cathode material to be used in the manufacture of organic light emitting diode is selected from metals having a relatively low melting point that are available in thermal evaporation.
In addition, the cathode material is chosen depending on whether the light is emitted through the substrate or in an opposite direction of the substrate. In order to emit light through the substrate, the anode placed on the substrate is selected from materials having high transparency, and the cathode is selected from materials having high reflectivity, and thus the light emitting direction is induced from the opposite direction of the substrate toward the substrate. As the cathode suitable for this purpose, the above mentioned aluminum is generally used. On the contrary, in order to emit light in an opposite direction of the substrate, a transparent anode 22 is deposited on a material 21 having high reflectivity or a material having higher reflectivity and high work function (>4.5 eV) is used as the anode, as shown in FIG. 16. At the same time, as shown in FIG. 15, a material having lower reflectivity is formed as a thin film to form the cathode 43, in which in order to prevent surface plasmon that occurs on the thin film cathode or improve conductivity of the thin film cathode, a high transparent dielectric material layer or transparent conductive material layer 44 is formed on the cathode at a proper thickness to increase transparency. The cathode used for this purpose includes magnesium or magnesium-containing alloy, and the high dielectric or transparent conductive material includes metal oxide, oxide of metal mixture, silicon oxide, and silicon nitride, but is not limited thereto.
In order to increase the light intensity per unit area, two or more structures of organic light emitting diode are deposited on one substrate to manufacture a device having a multilayer structure. Such device is characterized in that two or more structures of organic light emitting diode are connected in series, and it includes two external electrodes (anode and cathode) and a charge generation layer interposed between units of the repeating organic light emitting diodes. Such device shows the characteristics that the light intensity per unit area is increased, and the driving voltage increases in proportion to the number of repeating unit, as compared to the general organic light emitting diode, but the current decreases in inverse proportion, so as to improve durability of the device.
As mentioned above, organic light emitting diodes may have a different structure from each other, and therefore, different materials can be employed. However, the devices have the different structure, but some common properties. That is, the common properties are to need a substrate having a mechanical strength suitable for the manufacture of organic light emitting diode, to require two or more electrodes having different polarity on the substrate, and to dispose thin organic material layers having electron transport and light emitting properties between two different polarity electrodes. Two different polarity electrodes are generally divided into cathode and anode, and each of them functions to inject electrons and holes into the organic materials.